Water swellable polymer particles are in general known and have numerous applications. Such particles may be used as a thickening agent for aqueous media, for example as a thickening agent in aqueous coating compositions.
Water swellable polymer particles can also be used as precursors in the formation of polymer particles comprising an internal void(s). Such particles are often referred to in the art as “vesiculated polymer particles”, and these too can be employed in a diverse array of applications.
For example, vesiculated polymer particles can be employed as opacifiers in coating compositions such as paints. Opacifiers are important components of paints, having the primary function of scattering light incident on an applied paint film. How well a paint film is able to visually obliterate a surface over which it is applied is referred to as its opacity. Titanium dioxide pigment is traditionally used as the main opacifier in paint formulations and it, together with the polymeric binder of the formulation, are the two main contributors to paint formulation cost. In the formulation of low sheen and flat paints, mineral extender pigments such as calcite, clay or talc are often incorporated in paint formulations to reduce specula reflection down to the desired level.
With the aim of reducing cost, mineral extenders may be added to a paint formulation at such a level that there is insufficient polymeric binder to bind (space fill) all the pigment present. The term “critical pigment concentration” (CPVC) is often used to describe the point where complete space filling can no longer occur. The addition of mineral extender beyond the CPVC can therefore lead to the formation of air voids in the paint film as drying occurs. These voids scatter light in their own right and contribute to paint film opacity thereby allowing an opportunity to reduce the level of titanium dioxide and still achieve acceptable opacity or coverage. The accompanying formula cost saving, however, is at the expense of other paint film properties such as scrub resistance and stain resistance. In the case of stain resistance, the problem is that of stains penetrating into the voids in the film (film porosity).
Vesiculated polymer particles have been used in paint formulations to great effect by providing voids of air in paint films without the disadvantage of film porosity. In addition to providing means to reduce the amount of titanium dioxide required in paint, the use of vesiculated polymer particles can also have the benefit of reducing the amount of polymeric binder solids required to form a well integrated paint film.
Vesiculated polymer particles are often prepared in the form of an aqueous dispersion using suspension and emulsion polymerisation techniques. When in the form of an aqueous dispersion, the voids of the particles are typically filled with water. When such a dispersion is dried, for example as part of a paint formulation applied as a film, the voids of the particles should become filled with air and thus enhance the opacifying properties of the particles.
One approach to preparing vesiculated polymer particles involves first preparing water swellable polymer particles having a core of water swellable polymer and an outer shell or sheath of water permeable and typically non-swellable polymer. The polymer particles are then swollen with water such that the core polymer expands in volume and the shell accommodates this expansion in volume by in essence stretching. The swollen polymer particles can then be dehydrated such that the volume occupied by the swollen core polymer is reduced (i.e. deswells) without a significant reduction in the volume of the stretched shell polymer to thereby afford an internal void within the particle.
Methods used to prepare vesiculated polymer particles in this way are often complex. A particular challenge in preparing the water swellable precursor polymer particles has been to gain sufficient control over the polymerisation process to consistently afford polymer particles having uniform morphology.
Attempts have been made to use conventional free radical polymerisation processes to form water swellable polymer particles that may be used to prepare vesiculated polymer particles. However, such processes are prone to forming polymer particles having a non-uniform shell encapsulating the core water swellable polymer, which can in turn result in rupture of the shell as the core polymer swells thereby resulting in the formation of defective vesiculated polymer particles.
For the efficiency and reliability of products comprising water swellable polymer particles or vesiculated polymer particles formed therefrom, it is generally desirable that the particles are produced with a substantially uniform structure relatively controlled in reproducible manner.
Accordingly, there remains scope for improving on the prior art techniques for preparing water swellable polymer particles that are suitable for use in preparing vesiculated polymer particles, or at the very least to provide an alternative method for preparing such particles. It would also be advantageous to provide water swellable polymer particles with further functionality.